FIG. 1(a) is a sectional view of a transmission-type wavelength division multiplexer (WDM) according to a prior art. As shown in FIG. 1(a), the WDM 11 contains a fiber pigtail 111, a GRaded-INdex (GRIN) lens 112, a filter 113, a second GRIN lens 119, and a second fiber pigtail 114, all adhered together with one another by an adhesive 118. A common input fiber 115 and a reflection output fiber 116 are extended from the fiber pigtail 111 and a transmission output fiber 117 is extended from the second fiber pigtail 114. In a typical operation of the WDM 11, a multi-wavelength light signal shoots into the WDM 11 via the common input fiber 115 and passes through the GRIN lens 112. The filter 113 is designed to allow only lights within a specific band of wavelength to pass but reflect lights outside the specific band of wavelength. Therefore, the lights within the specific band of wavelength pass through the filter 113 and are coupled to the transmission output fiber 117 via the GRIN lens 119. The reflected lights on the other hand are coupled to the reflection output fiber 116 via the GRIN lens 112. For this type of WDM 11, the input light signal passes through the filter 113 only once and has a moderate adjacent channel isolation about 30 dB.
FIG. 1(b) is a sectional view of a reflection-type WDM according to a prior art. As shown in FIG. 1(b), the WDM 12 contains a fiber pigtail 121, a cannular spacer 122, a GRIN lens 123, a filter 124, and a reflection mirror 125, all adhered together with one another by an adhesive 129. A common input fiber 126, a reflection output fiber 127, and a transmission output fiber 128 are extended from the fiber pigtail 121.
In a typical operation of the WDM 12, a multi-wavelength light signal shoots into the WDM 12 via the common input fiber 126 and passes through the GRIN lens 123. The filter 124 has a multi-layer dielectric interference coating which allows only lights within a specific band of wavelength to pass, but reflects lights outside the band of wavelength. The lights within the specific band of wavelength therefore pass through the filter 124 and are reflected by the reflection mirror 125 to pass through the filter 124 one more time. The GRIN lens 123 then focuses the lights to the transmission output fiber 128 of the fiber pigtail 121. The lights outside the specific band of wavelength are reflected by the filter 124 and focused by the GRIN lens 123 to the reflection output fiber 127 of the fiber pigtail 121.
For this type of WDM 12, lights are filtered twice by a single filter 124 to achieve significantly higher adjacent channel isolation. Moreover, as only a single GRIN lens 123 and a single fiber pigtail 121 are required, the WDM 12 has a much smaller form factor. However, it should be noted that the angle of incidence for lights entering into the filter 124 is inversely proportional to the transmitted wavelength of lights allowed to pass through the filter 124. More specifically, the larger the angle of incidence is, the shorter the transmitted wavelength gets, and the smaller the angle of incidence is, the longer the transmitted wavelength gets. Therefore, in real life application, the reflection-type WDM should be designed in a way that the lights will have identical angle of incidence when passing through the filter twice. In this way, the transmitted wavelength will not get shorter due to the lights' two passage through the filter and canceling out each other. In addition, another important design consideration for the reflection-type WDM is to have a structure that can minimize the insertion loss both at the transmission output and the reflection output. A smaller insertion loss implies that less amount of energy is lost during the lights' travel through the WDM.